My music theory teacher in college always drew the distinction between key and mode. His point was that there is not, properly speaking such a thing as a C major key or a C minor key. He said that key refers to the central pitch, or the root note, only. From the root note, or key, you build a scale or mode. So he would refer to what we call the C major key as "the key of C in the Ionian mode" and what we refer to as the C minor key as "the key of C in the Aeolian mode."

It's worth noting, as the Wikipedia article Wheat mentioned does, that there are modes beyond the seven formed by starting at different points in C major. Some of these come from folk or cultural traditions (e.g. Hava Nagila is in a mode called Freygish) while others are more theoretical (e.g. the Bohlen-Pierce scale). OK, perhaps it wasn't worth noting the theoretical ones ;-)
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dumbledadOct 2 '12 at 18:02